Various methods are known as a grinding method of resin particles, particularly, fluorine resin particles. Among them, the impact grinding method with a pneumatic classifier is widely employed since the method is economically efficient in preparing resin particles of a relatively large size. However, when fluorine resin particles are ground according to the impact grinding method, the obtained ground particles have small apparent density, become fibrous and have inferior properties owing to the heat generation during the grinding process. Alternatively, jet grinding methods have been attempted to improve the properties of the ground particles, which comprises jetting compressed air toward the central axis of the grinding chamber through three opposed jet nozzles provided in the grinding chamber while resin particles to be ground are fluidized and continuously supplied from the top or bottom of the grinding chamber, thereby colliding the resin particles with each other to grind them (JP-A-63-194750, JP-A-64-4401, JP-A-4-271853, JP-A-6-254427 and JP-A-7-275731).
However, when fluorine resin particles or other types of resin particles are ground according to the grinding method using a conventional jet mill, the ground resin particles or non-classified coarse resin particles tend to become adhesive and agglomerate. The fallen particles sometimes remain in a bulk at the bottom of the grinding chamber, resulting in the reduction of grinding ability (amount of collected resin particles of an intended particle size). In this way, conventional jet mills have a disadvantage that the grinding ability is extremely low when compared with the impact grinding method.
In addition, when the temperature of the resin particles to be ground, the compressed air or the jet mill is high, the resin particles tend to have large elasticity and the grinding of the particles becomes difficult, resulting in the lowering of the grinding efficiency. Under such circumstances, various attempts have been made. For example, the resin particles to be ground are cooled, a cooling jacket is provided on the jet mill, and the compressed air for jetting is cooled beforehand.
Thus, a lot of attention has been paid when resin particles are ground by using a jet mill.
In view of the above problems, the present inventors have conducted intensive studies and found a novel process for preparing uniform resin particles efficiently by using a jet mill, and completed the present invention.
An object of the present invention is to provide a process for preparing ground resin particles by modifying a jet mill with opposed fluidized bed to increase the grinding efficiency dramatically, thereby making operating conditions less tight and achieving a smaller device size and reduced running costs.
Another object of the present invention is to provide a novel jet mill suitable for the process of the present invention.